Known heat-sensitive stencil sheets used for stencil printing include those which comprise a thermoplastic film such as a polyester film, a polyvinylidene chloride film, or a polypropylene film and a porous substrate comprising a tissue paper or a nonwoven fabric mainly composed of natural fibers or synthetic fibers. The thermoplastic film and the substrate are laminated together using an adhesive.
However, these conventional heat-sensitive stencil sheets have not necessarily been satisfactory in sharpness of printed images and printing endurance. Various reasons for the insufficient sharpness of images can be considered, and one of them is a problem caused by a state of dispersion of fibers constituting the substrate. That is, since conventionally used tissue papers composed of natural fibers, synthetic fibers or mixtures thereof are partially non-uniform in dispersion of fibers, permeation of ink is apt to become uneven, and, particularly, permeation of ink is hindered by fibers aggregating on perforated portions of the film. As a result, defects such as blurring of prints and formation of white dots in solid printing occur in random or collectively. In order to solve these defects, it has been proposed to make the constituting fibers finer or reduce the basis weight of the fibers as much as possible.
The sharpness of images can be improved to some extent by employing finer fibers or reducing the basis weight of fibers of the substrate, but there are problems such as deterioration of strength and rigidity of stencil sheets. If the strength of stencil sheets is low, there occur such phenomena that stencil sheets are torn or stretched during printing, causing shift of printing position with increase of the number of printed copies. Moreover, if the rigidity of a stencil sheet deteriorates, the printing machine is jammed with the stencil sheet which is being carried in the printing machine or wrinkling of the stencil sheet occurs when the stencil sheet is wound around a printing drum, which causes deterioration of print quality.
One way to solve these defects is use of a screen gauze as the substrate. In the case of using the screen gauze, problem such as the above-mentioned formation of white dots do not occur and, besides, the stencil sheet is excellent in strength and rigidity because fibers are regularly arranged in the screen gauze.
However, due to the regular arrangement of fibers in the substrate, when the heat-sensitive stencil sheet is perforated by a thermal head, it rather interferes with regular heat generation pattern of the thermal head to cause formation of moire in printed images.
For the prevention of the moire, JP-A-9-48183 proposes to use a screen gauze comprising regular or irregular arrangement of a plurality of regions having different mesh number as a porous substrate of heat-sensitive stencil sheets. However, production of such screen gauze having a plurality of regions differing in the mesh number requires much labor. Furthermore, since percentage of aperture or fiber diameter of the substrate is different in every region differing in the mesh number, amount of ink passing through every region differs at the time of printing, and, therefore, transfer of the ink becomes uneven on the whole printed surface.